UH Manoa Researchers Solve Watercress Mystery

A team of researchers from the University of Hawaiʻi at Manoa‘s College of Tropical Agriculture and Human Resources (CTAHR) recently collaborated to solve the mystery of the decline of some watercress crops on Oʻahu. Led by Agent Steve Fukuda of the Cooperative Extension Service, a team of CTAHR experts began to explore what they thought were the most logical causes. When the mystery was solved, the culprit was the one of the least expected causes: a type of pathogen rare to Hawaiʻi and spread by an insect vector never before found in Hawaiʻi.Among the first colleagues Fukuda consulted were fellow Oʻahu CES agent Randy Hamasaki; Ray Uchida, director of CTAHR‘s Agricultural Diagnostic Service Center (ADSC); and Desmond Ogata, ADSC pathologist. The initial suspicion was that the decline could be related to a problem with the water supply, possibly excessive salinity. A group of faculty from CTAHR‘s Departments of Tropical Plant and Soil Sciences and Natural Resources and Environmental Management were consulted, drawing on their expertise in water quality, plant nutrient management, soil chemistry, salinity, and other pertinent areas. ADSC Director Ray Uchida helped coordinate the CTAHR teamwork and authorized dozens of soil, water, and plant tissue analyses, but no nutritional deficiency or toxicity was indicated.Concurrently, Ogata was engaging expertise from CTAHR‘s Department of Plant and Environmental Protection Sciences to explore the possibility that a disease pathogen was involved. Unfortunately, the checks for bacterial and fungal pathogens were fruitless.In the meantime, the watercress decline progressed, increasing in severity. The farm that initially was infected lost most of its crop, and the condition spread to a second farm, then to another. At the second farm, it took only two months from no apparent sign of the problem to greater than 50 percent crop loss. Despite attempts by researchers, experiments showed the condition to be irreversible. As plant nutritional causes and usual disease problems were successfully eliminated as the cause of the decline, the suspicion began to emerge that the problem might be caused by a phytoplasma, formerly called MLO or mycoplasma-like organism.Phytoplasmas are rare in Hawaiʻi, but researcher Wayne Borth had previously done work on one that infects a native Hawaiian plant. His expertise was called upon for this investigation. As Borth began his research, Fukuda and Hamasaki returned to the field. They knew that if a phytoplasma was the culprit, there had to be an insect vector spreading the disease. Exploring the margins of the stricken watercress paddy, Hamasaki spotted and collected an unfamiliar leafhopper. ADSC‘s insect diagnostic expert Dick Tsuda made a preliminary identification and passed the sample to colleagues at the Hawaiʻi Department of Agriculture who obtained confirmation from an expert on the mainland. The insect was identified as the aster leafhopper, vector of aster yellows. Neither the insect nor the pathogen had been known before in Hawaiʻi. When a few days later Borth‘s assay confirmed the presence of a phytoplasma in the sick watercress and some weed plants adjacent to the paddy, the investigative circle narrowed to a conclusion. Pending taxonomic identification of the pathogen by electron microscopy, the existence of aster yellows in Hawaiʻi is almost certain.The two CES agents quickly convened a meeting of watercress growers at which Ogata, Hamasaki, and Borth recounted how the cause of the disorder had suddenly come into focus. The team advised the growers on steps to take, which included aggressive destruction of infected crops and weeds, and vector control. HDOA staff are monitoring the vector, and the CTAHR team is planning further studies on integrated pest management approaches to restoring watercress production and preventing further spread of the disease. Aster yellows infect a very broad range of plants — about 300 species, including many common vegetable crops.This is just one story among many that have played out in one particular or another over the years, as CTAHR‘s research and extension faculty investigate problems affecting Hawaiʻi‘s crops. This time, thanks to a cooperative group of watercress farmers, there is hope that this potentially serious plant disease and its vector can be contained.*Additional CTAHR faculty involved in this story included Barry Brennan, Samir El-Swaify, Carl Evensen, Nguyen Hue, Ron Mau, James Silva, Janice Uchida, and Goro Uehara.**The aster yellows phytoplasma is a plant disease; it has no affect on human health, and there is no danger in eating watercress that might be infected with it. You can help Hawaiʻi‘s watercress industry survive this threat by continuing to buy and enjoy their island fresh product.